Ipv6 loopback summarization

[fasalmehmood]

Sorry I am new to ipv6 summarization and is trying to figure out the summary address for these 3 loopbacks 

1: 2001:db8:0::1/128

2: 2001:db8:0::2/128

3: 2001:db8:0::3/128

Reference CCNP ENCORE book page 234

Answer is : 2001:db8:0:0::/65 network

My confusion is why 65 instead of 126 because it is only the last 2 bits changing or different?

Another example from page 279

3 loopbacks to summarization again

1: 2001:db8::1/128

2: 2001:db8::2/128

3: 2001:db8::3/128

Answer is 2001::db8:0:0::/62

Can someone send a link with Good explanation and some practice questions to master ipv6 summarization?

[Seb Rupik]

Hi there,

Are you sure you have copied those example subnets, because in both although syntactically different the expanded values are the same, ie:

2001:db8:0::1  == 2001:db8::1

Reverse engineering from the answers you gave the component prefixes that fit would be:

2001:db8:0:0:1::/128
2001:db8:0:0:2::/128
2001:db8:0:0:3::/128

summarised by:
2001:db8:0:0::/65

...and...

2001:db8:0:1::/128
2001:db8:0:2::/128
2001:db8:0:3::/128

summarised by:
2001:db8:0:0::/62

Either you have copied the questions down incorrectly or you should submit some more errata to the already long list here:

https://www.ciscopress.com/store/ccnp-and-ccie-enterprise-core-encor-350-401-official-9780135262030

Play around with subnets here (https://subnettingpractice.com/ipv6_subnetting.html) using the summary prefixes given as the answer, and note how the component prefixes I specify above fit within them, when the format is exactly the same.

cheers,

Seb.